Currently, most of liquid crystal displays (LCD) are based on the twisted nematic (TN) scheme that require the use of polarized light. A typical direct-view LCD panel consists of many optical components, such as a back lighting system, a liquid crystal panel with addressing electronics sandwiched between two plastic polarizers, a colour filter mosaic for forming full colour displays, etc. Each component has limited optical efficiency. For example, the plastic polarizers absorb at least 50% of light. The addressing electronics has a certain size aperture that limits the amount of light passes through the liquid crystal cell. The colour filters absorb at least two thirds of the light. As a result, the efficiency of such directview liquid crystal panel is very low; only about 5% of total light is used.
In order to enhance the efficiency of the direct-view LCDs, several polarization recovering approaches have been proposed in the past. The first approach is based on the use of the Brewster angle effect to separate s- and p-polarized light, such as the art taught by M. F. Weber in the U.S. Pat. No. 5,422,756, by M. Suzuki in "Reflective polarizer sheet on the backlighting unit", SID'97 Digest, 813(1997), by H. Tanase et al in "A new backlighting system with a polarizer light pipe for enhanced light output from LCDs", SID'97 Digest, 365(1997). However, this approach suffers a poor angular performance and low extinction ratios due to the inherited property of the Brewster angle effect. As a result, the gain from converting unwanted polarization to the wanted polarization is partially lost because of the poor performance of the polarizers. No practical system based on this approach is available for the direct-view LCD market.
The second approach is to use reflective cholesteric liquid crystal polarizers, such as the art taught by D. J. Broer et al in "Reflective cholesteric polarizer improving the light yield of back-and side-Lighted flat panel liquid crystal displays", SID'95 Asia Display Digest, 735(1995), by D. Coates et al in "New applications of liquid crystals and liquid crystal polymers", SID'96 Eurodisplay Digest, 91(1996), and by L. Li et al in "A single-layer super broadband reflective polarizer", SID'96 Digest, 111 (1996). Although such polarizers may have a broad band, their extinction ratios are low, about 10:1 to 20:1. A second "cleanup" polarizer is required to absorb the unwanted polarized light and to bring the extinction ratio to a desired level greater than 100:1. In addition, the performance of such polarizers is sensitive to temperature and UV radiation.
The third approach is to use co-extruded reflective plastic polarizer, such as the art taught by A. J. Ouderkirk et al in "Reflective polarizer display," U.S. Pat. No. 5,828,488. The polarizer consists of a few hundred to a few thousand stretched films made of two plastic materials. One material has birefringence due to the stretching and the other does not. At normal angle of incidence, light polarized in one direction passes because the refractive indices of the two materials are matched. Light polarized in the other direction sees a refractive index difference because of the birefringence. As a result, this polarized light is partially reflected. Its reflectance depends on the refractive index difference as well as the number of layers. Since the refractive index difference is rather small, in order to achieve high reflectance over a broad band of wavelengths, a large number of plastic films with different thicknesses are required. This polarizer is broad band and wide angle. One disadvantage of this approach is that the extinction ratio is small. Second, there is some light loss due to absorption by the films and scattering at layer interfaces.
Therefore, the objective of this research is to develop a high efficiency polarizing light source, more particularly a back-lighting system for direct-view LCDs.